The present invention relates to a magnetoresistive effect element for obtaining a magnetoresistive change by causing a current to flow in the direction perpendicular to the layer plane and a magnetic memory device.
As information communication devices, in particular, personal small devices such as personal digital assistants are making great spread, elements such as memories and logics comprising information communication devices are requested to have higher performance such as higher integration degree, higher operation speed and lower power consumption. In particular, technologies for making nonvolatile memories become higher in density and larger in storage capacity are progressively increasing their importance as technologies for replacing hard disk and optical disc that cannot be essentially miniaturized because they have movable portions.
As nonvolatile memories, there may be enumerated flash memories using semiconductors and FRAM (Ferro electric Random Access Memory) using ferroelectric material and the like. However, the flash memory encounters with a drawback that its write speed is as slow as the microsecond order. On the other hand, it is pointed out that the FRAM has a problem in which it cannot be rewritten so many times.
A magnetic memory device called an MRAM (Magnetic Random Access Memory), which had been written in “Wang et al., IEEE Trans Magn, 33 (1977), 4498” receives a remarkable attention as nonvolatile memory which can overcome these drawbacks. Since this MRAM is simple in structure, it can be easily integrated at a higher integration degree. Moreover, since it is able to memorize information based upon the rotation of magnetic moment, it can be rewritten so many times. It is also expected that the access time of this magnetic random access memory will be very high, and it was already confirmed that it can be operated at the access time of nanosecond order.
A magnetoresistive effect element for use with this MRAM, in particular, a tunnel magnetoresistive (Tunnel Magnetoresistive TMR) element is fundamentally composed of a ferromagnetic tunnel junction of ferromagnetic layer/tunnel barrier layer/ferromagnetic layer. This element generates magnetoresistive effect in response to a relative angle between the magnetizations of the two magnetic layers when an external magnetic field is applied to the ferromagnetic layers under the condition in which a constant current is flowing through the ferromagnetic layers. When the magnetization directions of the two magnetic layers are anti-parallel to each other, a resistance value becomes maximum. When they are parallel to each other, a resistance value becomes minimum. Function of memory element can be demonstrated by creating the anti-parallel state and the parallel state with application of the external magnetic field.
In particular, in the spin-valve type TMR element, when one ferromagnetic layer is antiferromagnetically coupled to the adjacent antiferromagnetic layer and thereby the magnetization direction is always made constant, thereby resulting in the same being placed in the state of a magnetization fixed layer. The other ferromagnetic layer is formed as an information recording layer of which the magnetization direction can be easily inverted with application of external magnetic field and the like.
This resistance changing ratio is expressed by the following equation (1) where P1, P2 represent spin polarizabilities of the two magnetic layers.2P1P2/(1−P1P2)  (1)
As described above, the resistance changing ratio increases as the respective spin polarizabilities increase. With respect to a relationship between materials for use with ferromagnetic layers and this resistance changing ratio, ferromagnetic chemical elements of Fe group such as Fe, Co, Ni and alloys of three kinds thereof have been reported so far.
The MRAM is fundamentally composed of a plurality of bit write lines, a plurality of word lines intersecting these bit write lines and TMR elements provided at crossing points between these bit write lines and word write lines as magnetic memory elements as has been disclosed in Japanese laid-open patent application No. 10-116490. Then, when information is written in such MRAM, information is selectively written in the TMR element by utilizing an asteroid characteristic.
The bit write line and the word write line for use with the MRAM are made of conductive thin films such as Cu and Al which are interconnection materials of ordinary semiconductor devices. When information is written in a magnetic memory element of which the inverted magnetic field, for example, is 200 Oe by the bit write line and the word write line made of such ordinary interconnection materials, the bit write line and the word write line being 0.25 μm in width, a current of approximately 2 mA is required. When the bit write line and the word write line have a thickness of 0.25 μm that is the same as the line width thereof, a current density obtained at that time is 3.2×106 A/cm3 that is close to approximately a limit value of breaking of wire by electromigration. Accordingly, reduction of the write current is indispensable for maintaining reliability of interconnection. Moreover, in view of a problem of heat generated by the write current and from a standpoint of decreasing power consumption, this write current has to be decreased.
As a method of realizing the reduction of the write current in the MRAM, there is enumerated a method of decreasing a coercive force of the TMR element. The coercive force of the TMR element is properly determined based upon suitable factors such as the size, shape, layer arrangement of the TMR element and selection of materials. However, when the TMR element is microminiaturized for the purpose of increasing a recording density of the MRAM, for example, a disadvantage occurs, in which the coercive force of the TMR element increases. Accordingly, in order to microminiaturize (to increase integration degree) of the MRAM and to decrease the write current at the same time, the decrease of the coercive force of the TMR element should be attained from the materials standpoint.
If the magnetic characteristic of the TMR element is dispersed at every element in the MRAM and the magnetic characteristic is dispersed when the same element is measured repeatedly, then a problem arises, in which the selective writing using the asteroid characteristic becomes difficult. Therefore, the TMR element is requested to have a magnetic characteristic by which an ideal asteroid curve can be drawn. In order to draw the ideal asteroid curve, an R-H (resistance-magnetic field) curve obtained when TMR is measured should not have noise such as a Barkhausen noise, a rectangle property of a wave form should be excellent, the magnetization state should be stable and the dispersion of the coercive force Hc should be small.
Information may be read out from the TMR element as follows. When magnetic moments of one ferromagnetic layer and the other magnetic layer across the tunnel barrier layer are anti-parallel to each other, this state is referred to as a “1”, for example. Conversely, when the respective magnetic moments are parallel to each other, this state is referred to as a “0”. Information is read out from the element based upon a difference current obtained at a constant bias voltage or a difference voltage obtained at a constant bias current in these states. Accordingly, when scatterings of resistance between the elements are the same, a higher TMR ratio is advantageous and hence a memory that can operate at a high speed, having a high integration degree and having a low error rate can be realized.
Bias voltage dependence of the resistance changing ratio exists in the TMR ratio, and it is known that the TMR ratio decreases as the bias voltage increases. When information is read out from the element based upon the difference current or the difference voltage, since it is customary for the resistance changing ratio to take the maximum value of the read signal at the voltage (Vh) which decreases by half depending upon the bias voltage dependence, small bias voltage dependence is effective for decreasing read errors.
As described above, the TMR element for use with the MRAM should satisfy the above-mentioned write characteristic requirements and the above-mentioned read characteristic requirements at the same time.
However, when the materials of the ferromagnetic layers of the TMR element are selected, if the alloy compositions by which the spin polarizabilities shown by P1 and P2 in the equation (1) are increased are selected from materials made of only ferromagnetic transition metal chemical elements of Co, Fe, Ni, then the coercive force Hc of the TMR element generally tends to increase.
When the information recording layer is made of a Co75Fe25 (atomic %) alloy or the like, although a TMR ratio having large spin polarizabilities and which is greater than 40% can be maintained, it is unavoidable that the coercive force Hc also increases.
But instead, when the information recording layer is made of an Ni80Fe20 (atomic %) which is what might be called a permalloy that is known as a soft magnetic material, although the coercive force Hc can decrease, the spin polarizabilities are small as compared with the above-mentioned Co75Fe25 (atomic %) alloy so that the TMR ratio is lowered up to about 33%.
Moreover, although a Co90Fe10 (atomic %) alloy can produce a TMR ratio of approximately 33% and can suppress the coercive force Hc to approximately an intermediate value obtained between the above-mentioned Co75Fe25 (atomic %) alloy and the above-mentioned Ni80Fe20 (atomic %) alloy, this alloy is inferior in rectangle ratio of the R-H curve and is unable to provide the asteroid characteristic by which information can be rewritten in the element.